1. Field of The Invention
This invention relates to method and apparatus for processing color images which are adapted for use in ink jet printers for printing color images. More particularly, the invention relates to method and apparatus for processing color images whereby images of good quality can be printed, without any bleeding, on ordinary paper sheets, such as paper sheets for xerography, ordinarily used in electrophotographic copying machines.
2. Description of the Related Art
As is known in the art, ink jet printing methods comprise discharging and flying ink droplets from a nozzle of a discharge energy generator, such as an piezoelectric device or an electrothermal conversion device, used as a discharge drive source, and permitting, as shown in FIG. 26, ink droplets 101 to be deposited on and absorbed in a material 100 to be printed, such as a printing paper sheet, film or cloth, thereby printing images on the material 100. These methods are advantageous in that they can be performed at low noises without use of any specific fixing treatment and that not only black-and-white images, but also full color images can be printed.
In order to carry out full color image printing using the ink jet printing method, it is usual to employ printing sheets for ink jet printing, which have a coating layer applied onto a base sheet and made of a dispersion of fine particles of SiO.sub.2 or CaO in a hydrophilic resin, in order to permit good absorption and fixing of an ink in a material to be printed and to obtain images of high quality. This eventually leads to a high so-called running cost which is a cost for obtaining one print.
On the other hand, in the ink jet printing method, when if full color images are printed on materials having no good ink absorptivity, such as sheets for xerography or wood-free paper sheets usually employed in electrophotographic copying machines, not on printing sheets adapted for use in ink jet printing, the following problems will arise. For full color image printing, where paper sheets for xerography or wood-free paper which is not good at ink absorptivity is used, as a material to be printed, in combination with an ink of the type which can yield letter or line images of relatively good quality and which is not high in drying speed, ink droplets 101 on a material 100 to be printed do not dry immediately as is shown in FIGS. 27a and 27b. In the case, the ink droplets 101 run and spread to an extent that adjacent printed ink droplets are associated with one another. As a consequence, so-called color bleeding takes place between adjacent ink droplets having different colors, thus presenting the problem that the colors are undesirably mingled, resulting in the degradation of image quality.
On the contrary, where paper sheets for xerography or wood-free paper which is not good at ink absorptivity is used, as a material to be printed, in combination with an ink of the type which is relatively high in drying and infiltration rates, it will be possible to suppress the bleeding between the color inks in full color images. However, as shown in FIGS. 28a and 28b, ink droplets 101 readily, deeply infiltrate into the material to be printed without leaving the color stuffs on the surface of the sheet. The printed region of the ink droplets 101 is low in concentration with a narrow color reproduction range. In addition, the ink is dried as spreading horizontally relative to the surface of the material to be printed, with the attendant problem that letter and/or line images become relatively thick with and the image quality becomes degraded owing to the feathering.
As will be apparent from the above, in the prior art ink jet printing method, if there is used a material to be printed having a slow ink absorption other than printing paper only for ink jet printing, it has been very difficult to attain a high quality of both full color images and letter and/or line images. To avoid this, printing paper only for ink jet printing has to be used as a material to be printed, resulting in a high running cost, i.e. a cost for obtaining one print. This is one of obstacles which have to be overcome for permitting wide use of ink jet printing devices.
In order to overcome the above difficulty, a number of usable techniques have been already proposed.
For instance, with an ink jet printer disclosed in Japanese Patent Laid-open No. 4-355157 wherein plural, different color inks are used for color image printing, the inks have different ink compositions so that at least one ink has an infiltration into a printing medium different from an infiltration of the other inks.
Japanese Patent Laid-open No. 4-364961 discloses an ink jet printer wherein different color inks from a plurality of printing means are used for color image printing, the ink compositions are so arranged that at least one ink has a fixing property different from that of the other inks. The ink jet printer proposed in this application includes such an arrangement that an ink to be precedently printed has a fixing rate higher than an ink which is subsequently printed.
In the above ink jet printing, preference is made to the quality of color images and the quick-drying property of the color inks is enhanced thereby preventing the occurrence of a so-called color bleeding, i.e. a bleeding between colors to ensure good-quality color images being printed. In view of this, attempts have been made to increase the infiltrating power of the inks by adding, to the inks, surface active agents as set out in Japanese Patent Publication No. 62-11781 and block copolymers of propylene oxide and ethylene oxide proposed by us (Japanese Patent Laid-open No. 4-325574).
Thus, where quick-drying inks are used for all colors, the respective inks are mostly infiltrated toward the direction of depth of the sheet as shown in FIG. 28b. This results in the lowering of a print density on the surface of the sheet and also in the degradation in quality of letter and/or line images printed in black ink. In color ink jet printers wherein cyan, magenta and yellow color inks are used in combination with a black ink, it is especially necessary to print letters of a high print density, under which usual practice is that only a black ink used has a low infiltration rate.
However, the prior art techniques have the following problems. Such a difference between the infiltration properties as set forth in the ink jet printer of Japanese Patent Laid-open No. 4-355157 undesirably places a limitation with respect to the reconciliation between color images which are free of any bleeding between different color image regions and sharp images.
On the other hand, with the case of the ink jet printer disclosed in Japanese Patent Laid-open No. 4-364961 wherein at least one ink is prepared as having a fixing property to a printing medium different from that of the other inks and wherein an ink which is precedently printed has a fixing rate higher than an ink to be subsequently printed, the infiltration speed of the subsequently printed ink whose fixing rate is slower greatly differs in between the printed portion and a print-free portion. The ink subsequently printed is drawn non-uniformly toward the printed portion, thereby causing the color bleeding. This has been experimentally confirmed by us.
In the known ink jet prints, where there are used color inks which have, respectively, a high infiltration rate and a black ink whose infiltration rate is low, the following problem arises. When an image region of the color ink having a high infiltration rate and an image region of the black ink having a low infiltration rate are contacted with each other on ordinary paper, the black ink with a low infiltration rate diffuses toward the color ink region as shown in FIGS. 27a and 27b, thereby causing color mingling or bleeding between the black ink and the color ink to lower the image quality.